Recent advances in the research on the interaction of ultrashort laser pulses with matter have shown that the outcome of many processes depends on the relative phase between the pulse envelope and the carrier wave, also called carrier envelope phase (CE-phase), see G. G. Paulus et al. in “Nature”, vol. 414, 2001, p. 182; A. Baltuska et al. in “Nature” vol. 421, 2003, p. 611; and A. Baltuska et al. in “IEEE J. QE” vol. 9, 2003, p. 972. Control over the CE-phase of few-cycle pulses allowed studying processes on time-scales shorter than the optical cycle, opening the door to attosecond metrology (1 as =10−18 s) and creating a new research field in physics dubbed ‘attoscience’, see R. Kienberger et al. in “Nature” vol. 427, 2004, p. 817; and E. Goulielmakis et al. in “Science” vol. 305, 2004, p. 1267. These Experiments need control over the CE phase over long periods of time.
Two basic approaches have been described for stabilising the CE-phase. A. Baltuska et al. (“Nature” vol. 421, 2003, p. 611) have proposed a stabilisation setup, which is illustrated in FIG. 8. With this setup, the CE-phase of a chirped-pulse amplifier 20′ is stabilised using two control loops. In a first loop 40′ including an f-to-2f interferometer 41′ and locking electronics 42′, a seed oscillator 10′ is stabilised. In the second loop 50′ including another f-to-2f interferometer 51′, an offset is applied to the oscillator locking electronics 42′ in order to stabilise the CE phase at the output of the amplifier 20′. The phase stabilisation of the oscillator 10′ is forced to change the carrier envelope phase of the pulses seeded into the amplifier. This is achieved by changing the offset signal-value in the locking electronics 42′, which in fact causes a controlled phase slipping of the oscillator pulses. The phase drift of the oscillator 10′ is stabilised to be exactly π/2 between two pulses, ensuring that every fourth pulse coming from the oscillator to have the same phase. This is done by locking the beat signal to a quarter of the oscillator repetition rate. A frequency can be locked to another frequency with a fast ‘up-down’ counter, by letting the counter increment with every period of the reference frequency, and decrement with every period of the frequency to be stabilised. When the output value of the counter is filtered with a low-pass filter, an error signal is generated by comparing this value with a reference value. By changing the reference value, a controlled phase shift, proportional to the change of the reference value is introduced.
The stabilisation setup of A. Baltuska et al. has a first disadvantage as it exploits an additional degree of freedom of the oscillator phase locking electronics, potentially decreasing the quality of the lock. This decrease in quality of the lock can in fact be observed, and eventually causes the lock to break earlier than in the undisturbed case. A further disadvantage is related to the fact that the locking electronics 42′ is adapted to be operated with the signal from a single amplifier only. Efficient stabilising amplifier chains is excluded with the technique of A. Baltuska et al.
C. Li et al. have proposed another stabilisation setup (“Optics letters” vol. 31, 2006, p. 3113), which is illustrated in FIG. 9. Again, two control loops are used for stabilising the CE-phase of a chirped-pulse amplifier 20′, namely a first loop 40′ with the f-to-2f interferometer 41′ and locking electronics 42′ for stabilising the seed oscillator 10′ and a second loop 50′ with another f-to-2f interferometer 51′. Contrary to the technique of A. Baltuska et al., the second loop 50′ directly controls the amplifier 20′. The CE phase is stabilised by changing a distance of telescope gratings in a pulse stretcher 22′ of the amplifier 20′. As a first disadvantage, the technique of C. Li et al. is restricted to particular laser systems having a single amplifier only, which is operated with a grating based pulse stretcher. Grating based pulse stretcher represent complex optical systems, wherein each displacement of a grating causes additional undesired effects. Furthermore, the technique of C. Li et al. has an essential disadvantage in terms of the high sensitivity of stabilisation. Grating translation of about 1 μm yields a CE phase shift of more than 180°. Therefore, the practical control range of grating translation is restricted to about 2 μm or even smaller values, so that high precision drives are necessary for reliably stabilising the CE-phase.
An additional method has been demonstrated (M. Schätzel et al. in “Appl. Phys. B”, vol. 79, 2004, p. 1021) allowing to control the phase of few-cycle pulses. However, this method can not be applied to longer pulses, like those coming directly from amplifier systems, and it needs phase-stable input pulses to begin with.
It could therefore be helpful to provide an improved method of stabilising a carrier envelop phase of laser pulses, which method is capable to avoid the disadvantages of the conventional stabilising techniques. Furthermore, it could be helpful to provide an improved stabilising device for stabilising the carrier envelop phase of laser pulses avoiding the disadvantages of the conventional optical setups.